1. Field of the Invention
This invention relates to manufacturing methods and, more particularly, to a method, system, and storage medium for increasing the efficiency of integrated circuit manufacturing by improving manufacturing equipment utilization.
2. Description of the Relevant Art
Manufacture of integrated circuits (ICs) upon semiconductor substrates (xe2x80x9cwafersxe2x80x9d) involves numerous pieces of equipment (xe2x80x9cprocess toolsxe2x80x9d) and manufacturing steps. Evaluation of the quality of the ICs during manufacture is important to assure that the completed ICs will be functional. After each processing step, the quality of the ICs being manufactured could be evaluated; however, the number of steps is so large that it is not practical to evaluate after every step. Instead, evaluation occurs only after certain processing steps during manufacturing. Typically, not every wafer is evaluated but representative wafers are evaluated after a set number of wafers have been processed.
The evaluation of wafers typically includes making some type of measurement and then comparing the measurement result to an established acceptable range. The measurement can take many forms including inspection for quantity and type of defects on the surface of the wafer, measuring the response to electrical stimuli of various regions of the wafer, or determining the thickness and uniformity of a coating that has been deposited on the surface of the wafer. If the measurement result is found to be outside of the established acceptable range, one or more process tools are removed from use for manufacturing and these tools are referred to as being in the xe2x80x9cdownxe2x80x9d state. Conversely, any process tool that is available for use is referred to as being in the xe2x80x9cupxe2x80x9d state.
In general, the processing flow of wafers is controlled by an automated factory system. This system is connected to the computers controlling the individual processing tools used in the IC manufacturing and the system also maintains a database of the current operating state of each tool. The system can accept a measurement result and can compare the result to a predetermined acceptable range and automatically place a process tool in the down state if the result is outside of the acceptable range. An example of an automated factory system is WorkStream(trademark) sold by Consilium, Inc. of Mountain View, Calif.
Prior to a given measurement result being outside the acceptable range, many process tools may have been involved in the processing of the wafer. Multiple process tools therefore may have contributed to the result being outside the acceptable range. Some decision must be made as to which process tools to place in the down state. Generally, this decision is performed automatically by an automated factory system. The factory system is typically programmed to place the process tool most likely to have caused the result outside of the acceptable range in the down state. For example, certain types of defects on the wafer surface are often associated with a specific process tool. A potential difficulty occurs, however, if an incorrect process tool is placed in the down state. In such a case, the process tool in the down state, which is actually performing adequately, is not available for use while the process tool with an actual problem continues to produce defective ICs, thereby continuing to reduce efficiency and increase the cost of manufacturing.
During the time that a process tool is in the down state, personnel associated with the tool typically attempt to identify the problem and take corrective action. Once the problem is believed solved, the process tool may then be evaluated using test procedures, which may include usage of test wafers, to assure correct functioning before being placed back in the up state. A test wafer does not have functional ICs being manufactured upon it and, typically, is processed by only a single process tool and then evaluated to determine the functioning of that process tool.
The process tool, however, may not process test wafers in the exact same manner as product wafers, which are wafers upon which functional (sellable) ICs are being manufactured. The test wafers therefore may not be sensitive to a problem that could cause defective ICs to be manufactured. Although the test wafer may be found to be acceptable and the process tool placed in the up state, subsequent product wafers processed by the process tool may not be acceptable. It could be that the original problem, to which the test wafer is not sensitive, has not been corrected or that the original problem has been corrected but a new problem, to which the test wafer is not sensitive, has been created accidentally. For either case, defective ICs are produced resulting in reduced efficiency and increased cost of manufacturing.
Measurement results being outside the acceptable range can also be due to the integration or combination of manufacturing steps between process tools (often referred to as an xe2x80x9cintegration issuexe2x80x9d) instead of being due to a single tool. The state in which a process tool leaves a wafer can affect subsequent processing steps by other process tools. Any evaluation of a given process tool using a test wafer will not detect such a problem since the test wafer is processed by only the single tool. That process tool will then be placed back in the up state, but the original problem still may not have been addressed.
Measurement results being outside the acceptable range can sometimes cause process tools to be placed in the down state for problems that ultimately do not affect the functioning of the ICs. For example, certain process tools can induce defects on wafer surfaces in areas of the wafer that will not affect the performance of the completed ICs. These nuisance defects, however, will cause the process tool to be placed in the down state, necessitating personnel associated with the tool investigating for problems and then running test wafers before placing the process tool back in the up state. This results in a process tool that was performing adequately unnecessarily being removed from manufacturing for a period of time.
Individual process tools are also subject to routinely scheduled tests as an additional check on the tool""s performance. Such tests are typically performed using test wafers so that the performance of the individual tool may be isolated. If the process tool fails the test, it is immediately placed in the down state. During the time that the process tool is down, personnel associated with the tool attempt to determine the problem and take corrective action. Once the process tool passes the test it previously failed, the process tool is often placed in the up state. No evaluation is necessarily made at this time, however, to ensure that product wafers are being processed correctly. Although the original problem was corrected, a new problem or integration issue, to which the test wafer is not sensitive, could have been created accidentally.
If a process tool is repeatedly placed in the down state unnecessarily, there may be a reduction in the responsiveness and efficiency of the personnel associated with that tool. After a process tool is continually placed in the down state unnecessarily, for example, it may become routine for personnel to run a test wafer and place the tool back in the up state without even examining the tool. When an actual problem with the process tool occurs, the tool may be placed back into the up state if the test wafer is not sensitive to the problem. In this case, defective ICs will continue to be produced by the process tool.
Anytime a process tool is placed in the down state and thereby removed from production there is a decrease in efficiency and increase in cost of manufacturing ICs; however, a process tool producing defective ICs while remaining in the up state also causes a decrease in efficiency and increase in cost. It is also important to ensure that any process tool returned to the up state is not producing defective ICs. It is therefore desirable to develop an improved method of manufacturing control for removing the correct process tool or tools from production that are contributing to the production of defective ICs and ensuring that any process tool returned to production will not contribute to the production of defective ICs.
The problems outlined above are in large part addressed by a method in which process tools used in the manufacturing process have a third operating state, referred to as the xe2x80x9cconditionalxe2x80x9d state, in addition to the up and down states. Process tools that are suspected of causing results outside the acceptable range in measured product wafers may be placed in the conditional state. Process tools in the conditional state are allowed to continue processing product wafers albeit for a limited period whereas process tools in the up state are enabled to process product wafers and process tools in the down state are disabled from processing process wafers. While the process tools are in the conditional state personnel investigate to determine the exact tool or tools causing the problem. This investigation can take many forms including inspecting the measured wafers with results outside the acceptable range and processing test wafers on process tools in the conditional state. Tools determined to be responsible for the measurement results outside the acceptable range are then placed in the down state while all others are placed back in the up state.
Process tools are not allowed to remain in the conditional state indefinitely. If a determination of responsibility for an unacceptable measurement is not made before an expiration point is reached, the process tools are automatically placed in the down state. Typically, the expiration point is set to be either a time-out period of time or after a fixed number of additional product wafers are processed by the process tool. The expiration point is set relative to the risk of production of defective ICs. If a specific process tool is known to often produce defective ICs, the expiration point can be set very short to minimize the risk of defective ICs being produced or the expiration point can even be set to zero to eliminate the risk completely. Additionally, if a set number of additional product wafers are found to have measurement results outside the acceptable range before the expiration point is reached, the process tool may also be automatically placed in the down state.
A process tool placed in the down state is examined by personnel associated with the tool in an attempt to determine the problem and take corrective action. Once the problem is believed solved, the process tool is then evaluated using a test wafer. If the process tool passes this test it is placed up to the conditional state; otherwise, it remains in the down state. After passing a test with a test wafer, a process tool placed in the conditional state is then allowed to process a set number of product wafers. If results from the measurement of these wafers are evaluated to be acceptable, the process tool is placed in the up state; otherwise, the tool remains in the conditional state. Wafers which cause a process tool to remain in the conditional state may be due to a new problem being accidentally created on the tool or may be due to the integration of manufacturing steps between process tools. If the process tool is determined to still be the problem, it is placed back in the down state for further examination and corrective action. If the problem is determined to be due to the integration of process steps between process tools, then the appropriate tools are placed in the down state for examination and corrective action.
If a process tool fails a routinely scheduled test, which isolates the performance of the tool, the process tool is placed in the down state. After the process tool has been subjected to examination and corrective action, the process tool is tested using the test it had previously failed. If the process tool now passes this test, it is placed in the conditional state. The process tool is then allowed to process a set number of product wafers. If measurement of these wafers yields results within the acceptable range, the process tool is placed in the up state; otherwise, the process tool is placed back in the down state for further examination and corrective action.
The conditional state allows personnel time to investigate and to determine the process tool or tools causing the problem. This provides an opportunity to prevent process tools that are functioning correctly from being placed in the down state and taken out of production. The limited period a process tool can remain in the conditional state limits the quantity of defective wafers produced in the event a tool is not functioning correctly. Additionally, if the cause of measurement results outside of the acceptable range is determined not to affect the functioning of the ICs, as in the case of nuisance defects, no process tool is unnecessarily taken out of production. Furthermore, requiring process tools that are in the down state to be placed in the conditional state after repair ensures that no process tool is allowed to process large quantities of product wafers until the tool has explicitly demonstrated it is functioning correctly for both test and product wafers. In this way process tools are kept in production as much as possible while minimizing the potential for production of defective ICs. Use of the conditional state may also improve the responsiveness and efficiency of personnel associated with a process tool. For example, a reduction in the number of times the process tool is placed in the down state that turn out to be unnecessary may increase the diligence exhibited by the personnel when that tool is placed in the down state. Increased efficiency and reduced cost of manufacturing are therefore believed to result.
In addition to the method described above, a system is contemplated herein. The system comprises a computer system, connected to process tools, in which measurement results of product and test wafers can be stored. A database, on the computer system, maintains a record of the operating states of process tools. A program executing on the computer system can compare a measurement result against a target result and make appropriate modification to the record of the operating state of process tools in the database. The target result comprises a predetermined acceptable range for the measurement result and the comparison includes determining if the measurement result falls within or outside the acceptable range.
A computer-readable storage medium is also contemplated herein. The storage medium contains program instructions which can be implemented by an executable unit to control process tools. The storage medium includes measurement data, target data, and evaluation data. The measurement data corresponds to measurement results of product wafers after certain processing steps while the target data corresponds to a predetermined acceptable range for measurement results. The evaluation data indicates whether a measurement results falls within or outside the acceptable range. The instructions cause the executable unit to select the operating state of a process tool based on the evaluation data.